Thermistor - cold idle speed increase

[82DMC12]

I've been helping another owner and with some idle issues and it reminded me of some tests I was doing on my own car last year when I had a high idle (now resolved, it was primarily due to incorrectly set throttle spool and linkage).

Regarding the thermistor, the shop manual says "At engine coolant temperatures below 59F, a thermistor located on the coolant distribution pipe, signals the ECU module to switch the regulator to a "wider" open position. This will increase the idle speed (fast idle) during the engine warm up period."

On my car, when the engine is cold, and cold ambient temp (lower than 59F anyway) I do get the "rhythmic revving" that seems common on the DeLorean engine. It will start, the engine will do that revving about once every 1.5 seconds, and this could last anywhere from 10 seconds to maybe 45 seconds. Then the idle smooths out and it sounds "normal".

Some owners say they "never have revving". Perhaps they live somewhere where they never start their car at such a cool ambient temp.

Is this revving what the manual is referring to? Nearly every other car on Earth will have a true "fast idle", maybe idling at 1,000 when cold until the engine warms up enough to drop to normal RPM's. It will then hold this pretty steady speed until close loop Lambda takes over where it maintains the same speed (approximately) but then sounds very slightly less even due to lambda controlling the mixture. All of this has been confirmed with multi meter and dwell meter.

I've never heard a DeLorean that has a steady fast idle when cold and then drop to a lower steady hot idle. They either all sound like hot idle all the time, or they do the rhythmic dance for a bit before settling in.

Any other observations or comments on this topic?

[Bitsyncmaster]

What the OEM idle ECU does is the thermistor just sets how closed the idle motor will go. In extreme cold the idle motor will only close a little below half way. Maybe at 32 deg. F it may close to 1/4 closed. At normal engine temp it still does not close all the way (maybe 1/8 to 1/16 still open). The ECU has a fixed closed loop idle at 775 RPM at all engine temps. So it would depend on a lot of things (mostly oil viscosity and throttle adjustments) if you would see a fast idle only because the idle motor is holding open more than 775 RPM needs.

The idle switch does not shut off that fixed 775 RPM. Only when the idle switch closes it makes the idle motor open faster so you don't get a bad undershoot of the idle RPM when your foot comes off the gas.

Your idle hunt when engine is cold and lambda has not gone closed loop is most likely a sticky idle motor when it is cold. I've found the smaller Volvo idle motors are much better (smoother) than the stock OEM unit. Most of us get idle hunt when the lambda first goes closed loop. That is due to the lambda ECU swinging the mixture which causes the idle ECU to fight those mixture changes which change the RPM. I have designed a new lambda ECU which stops that problem but it would not fix your hunt.

[AugustneverEnds]

My grandpa's '81 Mercedes 380SL would pulse idle for a minute or so when it was started from cold (we're in Upstate NY so plenty of opportunities to start the car below 59?) My D has it too so I have just always agreed with the notion that it's normal for K-Jet equipped vehicles to behave that way.

Surprisingly my '88 BMW with Bosch Motronic (EFI) has always had a slight pulse idle when dead cold so maybe the pulse is endemic to all Bosch fuel injection systems as they age.

[David T]

There are several systems that are active when the motor is cold. The vacuum advance, the CPR, the heated air intake, the ECU is in open loop, and the idle speed. None of them are connected to each other and they all act independently. Depending on how well balanced all of the cylinders are is what mainly affects the idle quality.

[glockworks21]

I love the description. "rhythmic dancing" I was going to ask about this.

My rhythmic dancing does not go away when warm.. same 1.5 second (may 50ish rpm) up and down. I actually think its worse when warm. I will check in morning..



??

[AugustneverEnds]

I love the description. "rhythmic dancing" I was going to ask about this.

My rhythmic dancing does not go away when warm.. same 1.5 second (may 50ish rpm) up and down. I actually think its worse when warm. I will check in morning..



??

If it's doing it when warm then I would start looking for vacuum leaks (on D's it's almost always comes to vacuum leaks )

My second vote would be the idle speed motor, could be a little sticky, nothing some carb cleaner and a cotton swab can't fix.

[SS Spoiler]

my idle sometimes climbs to 1500/2000 rpm
when warm, but if I turn my light off and on it
drops to normal.....total mystery...

[Timeless]

My engine had a bit of idle hunt then stuck idle ~2500RPM before a VOD service in December 2022 which I started a thread about here. Faulty thermistor was replaced as well as cap/rotor, vacuum lines, DPI injector seals, Also had a leaking idle speed motor which was replaced. Since the VOD repairs the engine no longer has rhythmic idling when idling - cold or hot. Granted, I live in S. FL but there are days where ambient temperature is near or below 59F and it still does not idle hunt upon cold start. (DMCH stage 2)

[82DMC12]

I've performed every test there is on my idle speed motor and it always checks out.

• check ohms between pins
• check for clack full open left/right with +12 either side
• observe valve open and slowly close when turning key to RUN
• Smoke test engine and there are zero leaks
• No advance at idle blah blah blah blah

I've also gently cleaned the inside of the motor. I'm pretty sure it's working as best as it can... however I do have a spare NOS black-painted Volvo ISM which I might throw on just to give it a try. Will update with what I find.

Also of course I have tested my thermistor every way possible and it's certainly working and making good connection at the ECU.

In short, I'm totally satisfied with my idle system but the rhythmic surge is interesting, and the statement in the workshop manual doesn't really explain what the result of opening the ISM will be (will it create the surging? Perhaps it will if the CPR is running at reduced control pressure which changes so many other things).